Fix backword-compatibility issue of non-versioned config file

According to the doc about `config.toml` of containerd:

```
If no version number is specified inside the config file then it is assumed to
be a version 1 config and parsed as such.
```

However, it's not true recently.
This will break the backward-compatibility in some environment.
This commit fixes this issue.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

services/server/config/config.go

@@ -98,6 +98,7 @@ func (c *Config) ValidateV2() error {
 	version := c.GetVersion()
 	if version < 2 {
 		logrus.Warnf("deprecated version : `%d`, please switch to version `2`", version)
+		return nil
 	}
 	for _, p := range c.DisabledPlugins {
 		if len(strings.Split(p, ".")) < 4 {